Multicoat paint systems used in the automotive OEM finishing sector nowadays consist in general of an electrophoretically applied primer, which provides protection from corrosion and stonechipping, and a subsequent surfacer coat, which provides protection from stonechipping and smoothens the surface. This surfacer coat is usually applied to the already-baked primer, and cured. An alternative possibility is to cure primer and surfacer coat jointly. Applied subsequently to the cured surfacer coat is a single-coat finish or a decorative two-coat finish composed of a color and/or effect basecoat, which is applied in one or more spray passes depending on the particular shade, and of a protective clearcoat, which is applied wet-on-wet to the basecoat. The single-coat finish or the basecoat(s) and the clearcoat is or are then jointly cured.
Increasingly, however, carmakers are concerned to reduce the coat thicknesses of the paint system and also the number of operations and also, where appropriate, the number of coats, without any attendant deterioration in the performance profile of the multicoat paint systems.
Accordingly, multicoat paint systems are known in which one or more basecoats are applied directly to an electrophoretically deposited primer.
Optionally, over the basecoat(s), a clearcoat is applied. Then the basecoat(s) and optionally the clearcoat are jointly cured. With this method, the surfacer coat that is normally situated between cathodic electrocoat and basecoat is omitted.
Thus WO2006/097201 describes a surfacer-less method of this kind for producing a multicoat paint system, in which a first pigmented basecoat film, a second pigmented basecoat film, and a clearcoat film are applied to a baked, cathodically deposited electrocoat primer, and all three coating films are jointly cured. In this case it is essential to the invention to ensure the UV protection of the cathodically deposited electrocoat primer by adding special light stabilizers in the first basecoat film. The clearcoat coating composition used may be any of the commonly used clearcoat materials, such as, for example, the commercial 2K [two-component] polyurethane clearcoat EverGloss® from BASF Coatings GmbH.
WO2010/060523 likewise describes a surfacer-less method in which in place of the surfacer, for the purpose of improving the appearance, particularly in the case of rough substrates, a pigment-free coating material is applied first to the electrocoat primer, before the basecoat films and the concluding clearcoat are applied. As clearcoat coating composition it is possible again to use all commonly employed clearcoat materials, such as, for example, the commercial 2K polyurethane clearcoat EverGloss® from BASF Coatings GmbH.
WO07/044769 discloses a method for producing a multicoat paint system by applying a first primer coating film, a second, pigmented basecoat film, and a clearcoat film, and jointly curing all three coating films. The primer coating film comprises at least one caprolactone-modified, linear acrylate resin, ensuring a good overall appearance to the resultant multicoat finish. As a clearcoat coating composition in the method it is possible to use all known clearcoat coating compositions; the working examples use a 1K [one-component] clearcoat coating composition based on a silanized acrylate resin.
WO2010/139375 describes multicoat paint systems where a conventional basecoat film and a clearcoat film are applied to a baked primer and a baked surfacer coat, and are jointly cured. A feature of the multicoat paint systems is that the clearcoat coating composition, as well as a hydroxyl group-containing compound (A), an isocyanate group-containing compound (B) and an amino resin (C), comprises at least one unsaturated cyclic, sterically hindered amine (D), for preventing the incidence of pops.
Also known from WO2007/137632 are multicoat paint systems where an aqueous basecoat film and a clearcoat film based on a 2K polyurethane coating composition are applied to a baked primer and a baked surfacer coat, and are jointly cured. With the aim of improving window bonding, the 2K polyurethane coating composition comprises an adhesion promoter in the form of a mixture of a phosphonic diester and/or a diphosphonic diester and at least one reaction product of a monoalkoxysilylamine and/or bisalkoxysilylamine with an isocyanurate, though this reaction product no longer has any free isocyanate groups.
Furthermore, WO 10/063332 discloses nonaqueous coating material compositions which comprise at least one polyhydroxyl group-containing compound (A), at least polyisocyanate (B) having free or blocked isocyanate groups and having silane groups, and/or the dimer and/or oligomer thereof, at least one catalyst (D) for the crosslinking of the silane groups, and at least one urea-based rheological assistant (R), and optionally, in addition, a rheological assistant based on fumed silica. For achieving a good overall appearance it is essential to the invention that the polyhydroxyl group-containing compound (A) used in these coating compositions is based on a hyperbranched, dendritic, hydroxy-functional polyester in which at least one hydroxyl group has been esterified with a C8 to C9 monocarboxylic acid. These coating compositions are applied wet-on-wet to a commercial aqueous basecoat material, and these two coating films are then jointly baked.
The resulting coatings have a high scratch resistance and weathering resistance and also at the same time a good appearance, although even lower short-wave values, as they are known, at increased film thicknesses of at least 40 μm are desirable. Further desirable in these coating compositions is an improvement in the sandability and polishability of the resultant coatings.
Lastly, international patent application PCT/EP 2012/059611, not yet laid open, and international patent application PCT/EP2012/058355, not yet laid open, describe multicoat paint systems where, atop a cured, electrophoretically deposited primer and a cured surfacer coat, a basecoat film and a clearcoat film are applied, it being essential to the invention that the clearcoat coating composition comprises not only a polyhydroxyl group-containing component (A) but also a compound (B) which contains isocyanate groups and silane groups and which more particularly is a mixture of a compound (B1) having a cycloaliphatic polyisocyanate parent structure and of a compound (B2) having an acyclic aliphatic polyisocyanate parent structure. In these applications, the addition of fumed silica-based rheological assistants to the clearcoat coating compositions is no more described than is the use of the clearcoat coating compositions in surfacer-less processes.